Variability of the Indonesian Throughflow thermal profile over the last 25-kyr: A perspective from the southern Makassar Strait

The Indonesian Throughflow (ITF), the sole conduit between the Pacific and Indian Oceans, regulates the heat and fresh water budgets between these oceans and plays a key role as the ‘warm water pathway’ in the global thermohaline circulation. In this study, we reconstructed a 25-kyr history of the ITF thermocline, based on well-dated, high-resolution (~200 yrs) records of Pulleniatina obliquiloculata δ18O and Mg/Ca from Core SO217-18515 (3.63°S, 119.54°E; water depth: 688 m), retrieved from the southern Makassar Strait. We integrated these new records with data from offshore Luzon (MD98-2188; 14.82°N, 123.49°E; water depth: 730 m), from the center of the Western Pacific Warm Pool (WPWP) (3cBX; 8.02°N, 139.6°E; water depth: 2829 m) and from the Timor Sea (MD01-2378; 13.08°S, 121.79°E; water depth: 1783 m). Both sea surface and thermocline temperatures decreased and their difference (ΔT) increased in the southern Makassar Strait and Timor Sea between 11 and 2.5 ka, corresponding to enhanced El Niño activity and dominance of East Asian winter monsoon over summer monsoon. In contrast, sea surface and thermocline temperatures increased and ΔT decreased during the last ~2.5 kyrs concomitantly with declining El Niño activity and relative weakening of the East Asian winter monsoon. Our results additionally show that ice volume corrected seawater δ18O of both sea surface and thermocline in Cores SO217-18515 and MD01-2378 co-vary with Borneo stalagmite δ18O, a proxy of regional precipitation in the WPWP, thus indicating vertical mixing of upper ocean waters along the ITF pathway. We propose that changes in the state of the El Niño-Southern Oscillation and in East Asian monsoon related to regional precipitation exerted a major influence on the ITF thermocline profile during the Holocene and largely controlled changes in ITF thermocline temperature throughout the last 25 kyrs.